Injection molding of plastic parts is a well known art. When the desired result of the molding process, however, is a strong pressure-resistant container having a top or neck opening of smaller diameter than that of the base, problems can arise. Due to the nature of the molding process, such a container must be molded in more than one part. Then, depending on the relative configurations of the molded parts, either the parts must be joined along seams running the length of the container, or a top portion must be joined to a bottom portion. Any joining of two such plastic parts can produce a weakened area along the join line and the longer the join line relative to the container, the greater the possibility of weakness.
Of all possible methods of joining two or more plastic parts, one of the strongest join-lines is produced by spin-welding. This method, which is in essence heat-welding produced by the friction of two circular rims turning against each other, can obviously only be utilized for or with thermoplastic and cylindrically-joined parts. Plastic parts of other configurations must be joined by other means.
Another type of weak point can result from the design of the molding mechanism itself. When plastic material is injected into a mold cavity at more than one position (through more than one side gate), the plastic material flows from each gate around a molding core. When the separate flows meet, one or more weld-line areas are formed. These areas or lines have different structural characteristics than does the rest of the plastic part. Usually the structural integrity of the part itself is weakest (more inclined to break or rupture) along such weld-lines.
The usual method of forming the body portion for such a container is to allow plastic material to flow around a core, producing a hollow cylindrical structure open at the bottom end and closed at the top end. If a mouth or neck opening into the container is desired, such an opening must then be machined into the non-closed end of the hollow cylindrical structure.
U.S. Pat. No. 1,052,966 to Bloomfield et al., "Pressure Vessels of a Spin-Welded Construction", discloses a container with an open neck, formed in two parts by injection molding and then spin-welded together. No details of the injection molding process or the method by which the neck opening is formed are given. It is assumed that either subsequent machining is necessary or that a side gating technique, which would result in body weld-lines is used. The essence of the invention technique is of spin welding itself as a superior means of bonding thermoplastic parts together.
U.S. Pat. No. 3,820,931 to Edwards, "Injection Mold Producing Open-Mouthed Thin-walled Containers Having Means to Prevent Radical Deflection of the Male Mold Part" discloses an apparatus and process for molding a container that has a single center gate opening into the top portion of the molding cavity. There is no disclosure or suggestion of the possibility of creating a neck opening in the molded container portion by any means.
U.S. Pat. No. 3,570,107 discloses a method for injection molding of a container portion having a lower open end. The essence of this invention is the method of exact sizing of the bottle body by means of a tapering core diameter. No method of removing a top neck sprue area to form an opening is disclosed or suggested: it is only stated that the top area of the bottle will be "subsequently removed," presumably by some form of machining.